Most optimized way of concatenation in strings

We always came across many situation on daily basis wherein we have to do tedious and very many string operations in our code. We all know that string manipulations are expensive operations. I would like to know which is the least expensive among the available versions.

The most common operations is concatenation(This is something that we can control to some extent). What is the best way to concatenate std::strings in C++ and various workarounds to speed up concatenation?